Import data from one form to another - C#

Hello guys.

I want to ask a relatively simple question, that is about importing data from one form to another.

So, I have two forms, the main one that contains a richTextBox and a button that says "Add". When I click the button, another form pops out and let me introduce some data in text boxes. I want to save that data into first form's richTextBox by clicking on "Save" in Form two.